Oxidation Dynamics of Epitaxial Rubrene Ultrathin Films

Great effort is being devoted to the fabrication of electronic devices based on rubrene thin films, because its outstanding charge transport properties make it one of the most promising organic semiconducting materials. Nonetheless, charge transport is strongly affected by the degree of crystallinity and degradation by photo-oxidation of rubrene. In the present work, in order to understand the dynamics of oxidation of rubrene when in the crystalline thin-film form, a combination of scanning probe techniques, such as Kelvin probe, phase contrast, and surface morphology atomic force microscopy, is used to study the oxidation process under ambient conditions of rubrene crystalline ultrathin films grown by organic molecular beam epitaxy on tetracene substrates. These films have a thickness of one or two molecular layers and consist of separated, island-like, epitaxial domains, whose orientation is determined by organic epitaxy. Theoretical calculations of the rubrene peroxide molecular dipole and structural data of rubrene and of purposely grown rubrene peroxide crystals, are exploited to determine the oxidation dynamics of such thin films and its connection with morphological, structural, and dielectric properties of the films. We demonstrate the formation of a native crystalline rubrene peroxide layer on top of the pristine rubrene crystalline domains. © 2011 American Chemical Society.